What is to be understood hereunder by the term “polyplanar imaging system” is an imaging system which for the recording of images has not just one swiveling plane but a plurality thereof, in particular two or three.
The present invention relates particularly to angiography systems as employed for medical interventions performed on the heart, for instance. Angiography systems traditionally produce simple x-ray projection images in which structures such as heart shadows, guide wires, catheters, and catheters filled with a contrast agent are discernible. Angiography devices of modern design typically have a C-shaped arm on one of whose ends an x-ray source and on the other of whose ends an associated x-ray detector is attached. The C-arm can be freely swiveled around a patient-supporting table, thereby allowing two-dimensional realtime x-ray images (fluoroscopic recordings) of the patient to be recorded from many different viewing directions. Rotating the C-arm around the patient will thus also allow angiography systems of such type to produce CT-like 3D images, referred to also as C-arm CT.
In numerous C-arm CT applications a contrast agent has to be injected during recording, lasting typically at least 4 to 5 seconds, in order to emphasize the structures of interest such as vessels or ventricles of the heart. The contrast agent must therein be present as far as possible in a temporally constant concentration during recording. So what matters is very good synchronizing between contrast agent injecting and image recording.
However, it is often not easy to keep the contrast agent for several seconds in precisely the structure intended to be imaged and at the same time prevent other structures from being contrasted. What is particularly problematic are the temporal dynamics associated with representing the left atrium. Said representing is a quite novel and very attractive method for supporting ablation therapies in the case of atrial fibrillation. A preferred protocol or, as the case may be, measuring protocol is concerned with the instant at which the contrast agent is injected into the main pulmonary artery and how quickly measuring takes place. Said agent is transported through the lung, and C-arm CT recording is started precisely when it has traversed the lung and reached the left atrium. The optimal instant for that is, though, different for each patient and not easy to determine.
One possible solution to the problem is to administer a test injection—which is to say a test bolus—prior to actual 3D recording. Quite a small amount of contrast agent is therein injected and the circulation time (which is the time from when the contrast agent is injected until it reaches the structure being examined) is then determined from fluoroscopic images or DSA recordings (DSA=Digital Subtraction Angiography).
However, that method poses various problems: Firstly, a substantial amount of contrast agent is used for it, typically 20 ml with 80 ml for 3D recording. Secondly, owing to the smaller amount of contrast agent the test bolus does not have the same dynamic properties as the subsequent, actual measuring bolus. Thirdly, the timing is often only poorly discernible from DSA recordings because they are projection recordings that do not adequately resolve the three-dimensional anatomy. Fourthly, there can be a difference between the test bolus and final recording owing to changes in circulation time (due to pulse and blood pressure variations).
In the prior art a biplanar C-arm x-ray system's second plane offers the advantage of being able to halve the recording time by having the two planes rotate simultaneously, with each only covering an angular range of approximately 100°. The problem with synchronizing then becomes even more critical, however, because the time for a rotation run is shorter. The term “rotation run” therein means that a C-arm is swiveled across an angular range of (at least) 180°+fan angle, with as a rule 50 to 500 images being recorded. The “fan angle” corresponds to the angle at which the x-ray source emits its x-rays and is as a rule approximately 20°.
Summarizing, it can be stated that the test-bolus method provides at best a very inadequate solution to the problem.